1. Field of the invention
The invention relates to forming metal leads, and more particularly to flexible techniques for excising and forming metal leads bonded to an electronic device which extend over a sidewall of the device.
2. Description of Related Art
In the manufacture of integrated circuits, silicon wafers containing many individual integrated circuits are fabricated and then the wafers are cut into individual integrated circuit chips or die. The chips are subsequently packaged and interconnected to other electrical components. During packaging, electrically conductive metal leads can be bonded between a chip and an interconnect substrate to provide proper electrical interconnection between different chips.
One technique for providing conductive interconnecting metal leads is tape-automated-bonding (TAB). The leads can be fabricated on a continuous carrier film in which the tape is a laminate of copper and a plastic insulator and the leads are etched in the copper while on the plastic carrier using well known etching processes. Or an additive metal fabrication can be used. The film is normally perforated along the edges with sprocket holes for use in advancement and alignment of the carrier film. Individual chips can be electrically bonded to individual lead frames in the tape, for instance by laser bonding as described in U.S. Pat. No. 4,845,335, and the lead frames can be removed or excised from the carrier film. Usually it is necessary to bend or form the leads prior to bonding the outer lead ends to an interconnect substrate.
Conventional excise and lead form of TAB devices requires the design and fabrication of custom tooling for each device size and/or lead form geometry. These tools basically conform to punch and die technology where the device is forced through a cutting die to excise it from a TAB film strip, and then forced around a forming die to produce the lead form. See, for instance, U.S. Pat. Nos. 2,997,907; 3,417,643; 4,064,917; 4,361,173; 4,371,012; 4,488,581; 4,559,978; 4,625,772; 4,633,920; 4,691,747; 4,787,426; 4,945,954 and 5,065,504.
As the pitch for TAB devices decreases (8 mils or less) it becomes more difficult to maintain the outer lead pitch during the excise and lead form operation. During excise sequence the device is literally punched through a cutting die. The cutting allowance is either designed in with a one-piece cutting die or adjustable with a four-piece cutting blade matrix. If this cutting allowance is incorrect, burrs will form and often cause lead splay.
Once excised from the TAB film strip, the leads become cantilevered beams supported by the inner lead bond and often a dielectric support ring. These lead beams have a very small cross-sectional area as compared to their length, making them susceptible to bending. As these beams are forced around a forming die, they are bent closest to the supported end first and continually abraded down their length until the second or lower bend is formed, making it difficult to control the pitch at the other end. Keeper bars (strips of dielectric on the lead tips) are often used to prevent splay on fine pitch devices; however, they may require more complex form tooling and present other problems during outer lead bonding.
In addition, due to the tight tolerances required on fine pitch devices, the punches and dies must be customized for the exacting specifications of each TAB design. As a result, the tools are expensive, require long lead times to procure, are labor intensive and are time consuming to install and tune. Once on line, this is an effective and relatively fast (e.g., 6 to 8 seconds) method of excise and lead form. However, less static activities such as the research and development of TAB multichip modules may be hindered.
Needless to say, there is a need for an alternative approach to TAB excise and lead form that allows an inexpensive set of hard tooling to be used for a variety of chip sizes and beam tape geometries while minimizing splaying and galling of the leads.